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ChestDemonstration data

V/Q Lung Scan (Ventilation-Perfusion)

The ventilation-perfusion (V/Q) scan compares regional pulmonary blood flow with regional ventilation to detect pulmonary embolism. Tc-99m MAA particles lodge transiently in pulmonary arterioles to map perfusion, while an inhaled radioaerosol or radioactive gas maps ventilation. A perfusion defect with normal ventilation (mismatch) in a segmental distribution is the hallmark of PE. It is the preferred test when CT pulmonary angiography is contraindicated, particularly in pregnancy and iodinated-contrast allergy, and delivers a lower breast/fetal dose.

Indications

  • Suspected acute pulmonary embolism when CTPA is contraindicated or undesirable
  • Pregnant or young patients where reduced breast and fetal radiation dose is advantageous
  • Iodinated contrast allergy or significant renal impairment
  • Diagnosis and mapping of chronic thromboembolic pulmonary hypertension (CTEPH)
  • Pre-operative quantification of regional lung function before lung resection or volume reduction

Contraindications & Cautions

  • Right-to-left cardiac shunt or severe pulmonary hypertension are relative contraindications for MAA (systemic embolisation of particles) — reduce particle number
  • Pregnancy is not a contraindication; a perfusion-only (half-dose MAA) protocol is often used first to limit fetal dose
  • Breastfeeding: interrupt or discard milk for ~12 h after Tc-99m aerosol/MAA per local guidance owing to free pertechnetate/aerosol
  • Known severe allergy to human albumin products (MAA)
  • Reduce MAA particle number in children, pregnancy and pulmonary hypertension to minimise capillary occlusion

Patient Preparation

  • Obtain a current chest radiograph (ideally within ~24 h) for correlation and interpretation
  • No fasting required; explain the breathing manoeuvre for the ventilation component
  • Confirm pregnancy status; counsel on the perfusion-only reduced-dose option where appropriate
  • Inject MAA with the patient supine to distribute particles evenly and reduce gravitational gradient
  • Ensure the patient can perform a tidal-breathing inhalation of the ventilation agent

Technique & Parameters

  • Perfusion: Tc-99m MAA ~40-150 MBq (~200,000-500,000 particles; fewer in pregnancy/pulmonary hypertension) injected slowly IV with the patient supine
  • Ventilation: Tc-99m Technegas or DTPA aerosol, or Xe-133 / Kr-81m gas, inhaled during tidal breathing
  • Acquire planar images in 6-8 projections (anterior, posterior, both laterals, and obliques) for both phases
  • When ventilation follows perfusion, use a higher ventilation dose to overcome existing Tc-99m activity (or vice versa)
  • SPECT or SPECT/CT improves segmental localisation and reduces indeterminate studies

Systematic Review

  • Correlate with the chest radiograph to identify matching parenchymal abnormalities
  • Assess perfusion images segment by segment for defects and their shape (wedge/segmental versus non-segmental)
  • Compare each perfusion defect with the corresponding ventilation image to classify as matched or mismatched
  • Count the number and size (large/moderate/small) of mismatched segmental defects
  • Apply modified PIOPED/PISAPED probability criteria to assign a diagnostic category

Key Findings & Significance

  • Segmental or larger perfusion defect with normal ventilation (V/Q mismatch) is the hallmark of acute PE
  • High probability: two or more large mismatched segmental perfusion defects without matching radiographic abnormality
  • Matched V/Q defect with corresponding radiographic opacity suggests parenchymal disease (e.g. pneumonia, COPD), not PE
  • Normal perfusion effectively excludes clinically significant PE
  • Multiple bilateral mismatched defects with mosaic perfusion in a stable patient suggest chronic thromboembolic disease (CTEPH)

Differential Considerations

  • V/Q mismatch: acute PE, chronic thromboembolic disease, extrinsic vascular compression, prior embolism/scarring
  • Matched defect: pneumonia, COPD/emphysema, atelectasis, effusion, bullae
  • Reverse mismatch (ventilation defect > perfusion defect): mucus plugging, consolidation, atelectasis
  • Non-segmental perfusion defect: cardiomegaly, mediastinal mass, pleural effusion
  • Diffuse heterogeneous perfusion: pulmonary hypertension, small-vessel disease

Pearls & Pitfalls

  • A perfusion-only study is often diagnostic in young patients with a normal chest radiograph and halves the dose
  • Interpret against a current chest radiograph — matched radiographic opacity reclassifies a defect away from high probability
  • The "stripe sign" (peripheral rim of preserved perfusion around a defect) argues against embolic occlusion
  • Inject MAA supine and avoid drawing blood back into the syringe to prevent hot-spot clumping artefacts
  • SPECT/CT reduces the intermediate/indeterminate category compared with planar imaging

Structured Report

  • State tracers, activities, projections and whether SPECT/CT was performed
  • Report the number, size and distribution of matched and mismatched defects with radiographic correlation
  • Assign a probability category (normal, very low, low, intermediate/indeterminate, high) using modified PIOPED
  • For CTEPH, describe the bilateral segmental mismatch pattern
  • Give a clear conclusion on the likelihood of PE and any recommended further imaging

References

  • Bajc M, et al. EANM guideline for ventilation/perfusion single-photon emission computed tomography (SPECT) for diagnosis of pulmonary embolism. Eur J Nucl Med Mol Imaging 2019
  • PIOPED Investigators. Value of the ventilation/perfusion scan in acute pulmonary embolism. JAMA 1990
  • SNMMI Procedure Standard for Lung Scintigraphy 4.0
  • Parker JA, et al. SNM practice guideline for lung scintigraphy. J Nucl Med Technol 2012

Educational clinical decision support only. Protocols vary by institution and equipment; always confirm with a qualified radiologist and local guidelines before clinical use.